The present invention relates to a method for measuring nitrogen oxide. Particularly, the present invention provides a method for measuring a very small amount of nitrogen oxide in a gas containing a great amount of interfering gas with a high preciseness.
With regard to a method for obtaining NOx amount in a gas to be measured by measuring an amount of oxygen generated by decomposing NOx in the gas to be measured, there has conventionally been, for example, a method disclosed in European Patent Laid Open No. 0678740A1 as shown in FIG. 13.
In FIG. 13, 80 is a NOx sensor having a main pump means 60 and an electric signal conversion means 70. The main pump means 60 includes an electrochemical pump cell 6 comprising a substrate 3 comprising an oxygen ion conductive solid electrolyte and an inner pump electrode 5 and an outer pump electrode 4 formed on the inner surface and outer surface of the substrate, respectively. The main pump means 60 treats oxygen contained in the measuring gas introduced from an external space by pumping processing on the basis of a control voltage applied between the inner pump electrode 5 and the outer pump electrode 4 by a power source 7. The electric signal conversion means 70 includes a pair of detection electrodes 11 and 14 formed on the side where the measuring gas that was treated by pumping processing by the main pump means is introduced. The electric signal conversion means 70 generates an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the measuring gas after treated by the pumping processing by the main pump means 60.
There has conventionally been used the NOx sensor 80 for measuring an amount of nitrogen oxide in a gas by introducing the gas to a first inner space 2 connected with the external space by means of a first diffusion resistance 1, controlling an oxygen partial pressure in an atmosphere in the first inner space 2 to be a predetermined low value at which NO is not decomposed substantially by the main pump means 60, introducing a controlled atmosphere of the first inner space 2 into a second inner space 9 connected with the first inner space 2 by means of a second diffusion resistance 8, and converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by the electric signal conversion means 70.
Incidentally, in FIG. 13, a potential difference detection means 40 measures an electromotive force V1 corresponding to a difference in oxygen partial pressure between an electrode 10 exposed in the first inner space 2 and an electrode 11 exposed in a space 15 where a standard gas is present. A voltage applied to the main pump means 60 is adjusted so that the electromotive force V1 is controlled to have a predetermined value.
However, it is difficult by this method to completely remove an influence of an interfering gas, for example, oxygen gas, carbonic acid gas, steam, or the like, in an exhaust gas of an internal engine. Because of a difference between a gas concentration in the side facing the first inner space 2 of an inner pump electrode 5 of the electrochemical pump cell 6 and that in the side of the outer pump electrode 4, a disorder of an atmosphere in the first inner space 2 by pulsation of a gas to be measured, an error caused by decomposition of water, a change of electrode activity or diffusivity of each gas component with a change of a temperature of a sensor element, a change of standing of electrodes and electrolyte, a discordance between an average value of an electrode potential of the electrochemical pump cell 6 and an average of a gas concentration, etc., it was difficult to maintain an oxygen partial pressure of a gas introduced into the second inner space 9 to be a certain value. When NOx in a gas having a high oxygen partial pressure is measured, an oxygen partial pressure of a gas introduced into the second inner space 9 is changed to be higher. According to the change, a base line of an electric signal (a pump current value Ip in FIG. 13) by an electric signal conversion means 70 corresponding to NOx amount is also changed. Particularly, it causes a large error when a very small amount of NOx is measured. On the contrary, when a steam amount of a gas to be measured increases, a hydrogen amount in the first inner space 2 in an equilibrium condition, a balance between hydrogen and oxygen in the second inner space 9 is lost by the difference in diffusion coefficients, a part of oxygen generated by decomposition of NOx is consumed by combining with hydrogen, thereby causing an error in measuring NOx.